The medical device industry produces a wide variety of electronic and mechanical devices for treating patient medical conditions. Depending upon the medical condition, medical devices can be surgically implanted or connected externally to the patient receiving treatment. Clinicians use medical devices alone or in combination with therapeutic substance therapies and surgery to treat patient medical conditions. For some medical conditions, medical devices provide the best, and sometimes the only, therapy to restore an individual to a more healthful condition and a fuller life.
One type of medical device is an implantable therapeutic substance infusion device. An implantable therapeutic substance infusion device is implanted by a clinician into a patient at a location appropriate for the therapy. Typically, a therapeutic substance infusion catheter is connected to the device outlet and implanted to infuse the therapeutic substance such as a drug or infusate at a programmed infusion rate and predetermined location to treat a condition such as pain, spasticity, cancer, and other medical conditions. Many therapeutic substance infusion devices are configured, so the device can be replenished with therapeutic substance through a septum while the device is implanted, so the time the device can be implanted may not be limited by therapeutic substance capacity. An example of an implantable therapeutic substance infusion is shown in Medtronic, Inc. product brochure entitled “SynchroMed™ Infusion System” (1995).
Other implantable devices exist which electrically stimulate neurological tissue to treat or relieve the symptoms of a wide variety of physiological or psychological maladies or pain. Such devices are typically part of systems that are entirely implantable within the patient or are partially implantable and partially external to the patient. Systems that are entirely implantable in the patient typically include an implantable pulse generator and an extension and lead or leads. In such a system, the implantable pulse generator, extension and lead are entirely implanted in the bodies of the patients. An example of such a system is the Itrel™ 3 system manufactured and sold by Medtronic, Inc. of Minneapolis, Minn. Because the implantable pulse generator is implanted, the power sources needed to power the implantable pulse generator are also implanted. Typically, the power source for an implantable pulse generator is a battery.
Each of these implantable devices delivers a therapeutic output to the patient. In the case of an implantable therapeutic substance infusion device, the therapeutic output can be a therapeutic substance which is infused into the patient. In the case of a neurological tissue stimulator, the therapeutic output is an electrical signal intended to produce a therapeutic result in the patient. Other types of implantable therapeutic delivery devices also exist including cardiac pacemakers and defibrillators.
Electrically powered implanted therapeutic delivery devices can require replacement once implanted due to factors such as battery consumption, corrosive damage and mechanical wear. Since replacement of the implanted therapeutic delivery device requires an invasive procedure of explanting the existing device and implanting a new device, it is desirable to only replace the therapeutic delivery device when replacement is required. Replacement of previously implanted therapeutic delivery devices was typically scheduled based upon a worst-case statically forecasted elective replacement period. The worst-case scenario typically resulted in the implanted therapeutic delivery device being replaced several months or even years before the implanted therapeutic delivery device actually required replacement.
Some previous implantable pulse generators such as pacemakers have monitored a single sensed battery condition to estimate replacement time for the implanted device or battery such as shown in U.S. Pat. No. 6,167,309, Lyden, entitled “Method For Monitoring End Of Life For Battery” (Dec. 26, 2000).
Battery monitors which monitor the voltage of the battery in order to determine, or to predict, the remaining longevity of the battery have an inherent shortcoming. The voltage of a battery will commonly very slowly decline over time with only a slight variation in the voltage until the voltage the battery nears the end of its useful life. As the battery nears the end of its useful life, the battery voltage will begin to decline at a greater rate, often dramatically. Such a battery is advantageous as a source of power for an implantable therapeutic delivery device because the battery delivers such an assured relatively constant voltage over most of the useful life of the device. However, such a battery creates a problem for a battery longevity monitor using the voltage of the battery in an attempt to determine the longevity of the battery. Since the battery voltage remains relatively constant over most of the life of the battery, it is difficult to predict whether the battery is in the early part of the relatively flat voltage curve or nearing the end of the relatively flat voltage curve. The difference, of course, is a marked difference in the predicted longevity of the battery.
The ability to accurately predict the remaining longevity of the power source of an implantable therapeutic delivery device enables the patient to receive maximum life from the device and minimize the frequency, and possibly the number, of explantation and reimplantation of the device simply for the replacement of the power source. Further, since some safety margin is usually built in and because the patient usually schedules any such explantation and reimplantation, often around a busy schedule, additional time off of the actual remaining longevity of the power source may be lost.
For the foregoing reasons, there is a need for an implantable therapeutic delivery device with active longevity prediction to increase the implantable therapeutic delivery device's effective life, reduce the need for a clinician to perform static longevity forecasts for therapy changes, facilitate elective replacement scheduling for the convenience of the patient and clinician, and many other improvements.